Various types of systems have been devised for inspecting oilfield tubulars. U.S. Pat. No. 5,600,069 discloses an improved system for the ultrasonic testing for oilfield tubulars having various diameters. Oilfield tubular non-destructive testing equipment ideally satisfies several basic objectives. The system should be operator friendly and capable of being used by various inspection personnel. An inspection system also is preferably compact and may be both manufactured and maintained at a comparatively low cost. Most importantly, the system should be accurate in detecting various types of flaws, and should maintain that accuracy when oilfield tubulars are passing at a high rate through the system.
Non-destructive inspection systems primarily intended for testing coiled tubing present unique challenges. Those skilled in the oil patch appreciate that coiled tubing is distinguishable from conventional oilfield pipe, casing, and tubing in that coiled tubing is wound on a drum at the surface then passed downhole. Although lengths of coiled tubing may be joined by various types of connections, a length of coiled tubing is typically thousands of feet, rather than being 30 feet in length which is conventional for "rigid" oilfield tubulars with threaded connections. Since coiled tubing is inherently flexible, it has significant advantages over conventional threaded end oilfield tubulars, including relatively low cost and reduced time to pass the tubing downhole. This flexibility for coiled tubing presents unique inspection problems since the relatively thin wall of the coiled tubing must meet predetermined specifications, and since the tubing diameter and ovality must be within selected limits to minimize the likelihood of a coiled tubing ruptures. Coiled tubing may undesirably tend to elongate when an excessive axial force is applied to the coiled tubing, thereby reducing the wall thickness and the tubing diameter. Also, oilfield tubing tends to experience longitudinal splits at a rate significantly in excess of longitudinal splits in conventional oilfield tubing with threaded connector ends.
Several coiled tubing inspection systems have been devised, but none of these systems test for all of the defects commonly associated with coiled tubing, and most of these prior art systems are not highly accurate at detecting defects in coiled tubing. An inspection system manufactured by Rosen Inspection Technologies in Lingen, Germany tests for coiled tubing wall thickness and material flaws, but the system is bulky and expensive. Moreover, the Rosen inspection system does not test for tubing diameter and ovality, nor is the system well equipped to detect longitudinal splits in coiled tubing. A separate unit downstream from the Rosen flaw detection inspection system is required to measure the length of the tubing being inspected. Other prior art coiled tubing inspection systems, including a system manufactured by Stylwan Inspection in Houston, Tex., use a relatively small number of detectors, and are even less accurate at detecting most coiled tubing defects. Inspection systems that are specifically intended to test for coiled tubing diameter or ovality have been devised, but these systems do not test for various other types of defects in coiled tubing, including wall thickness, flaws, and longitudinal splits.
The disadvantages of the prior art are overcome by the present invention, and an improved coiled tubing inspection system is hereinafter disclosed. The inspection system is particularly designed for simultaneously testing for various types of defects in coiled tubing, and has a desired high accuracy at detecting those defects when coiled tubing is passed at a high rate through the inspection system.